The present disclosure generally relates to a process for measuring and quantifying viscoelastic foam compression recovery times.
Flexible, viscoelastic polyurethane foam (also known as “slow recovery” foam,” memory” foam or “high damping” foam) is characterized by slow, gradual recovery from compression. While most of the physical properties of viscoelastic foams resemble those of conventional foams, the resilience of viscoelastic foams is much lower, generally less than about 15%. Suitable applications for viscoelastic foam take advantage of its shape conforming, energy attenuating and sound damping characteristics. For example, the foam can be used in mattresses to reduce pressure points, in athletic padding or helmets as a shock absorber, and in automotive interiors for soundproofing.
Currently, ASTM D3574 is available for measuring and categorizing various characteristics of urethane foam materials. Test M of the standard is directed to the measurement of total compression recovery time. In that measurement, a predetermined load is applied via a compression plate to the foam for a fixed amount of time, typically resulting in a significant indentation. Typically, the foam is compressed to 25% of its original height. The compression plate is then retracted to the original height and total recovery time, in seconds, is recorded once the foam contacts the compression plate, which corresponds to the original height.
However, one of the problems with testing viscoelastic foams in this manner is what is currently being referred to as “finger nailing”. Finger nailing is a phenomenon that has been observed during compression testing of viscoelastic foams where recovery to the original height is not constant. That is, recovery about a relatively shallow indentation caused by the contact with the compression plate is delayed for an extended period time whereas the bulk recovery of the compression to the finger nailing region can be relatively constant. As a result, the current test processes can result in an inaccurate total recovery time. For example, toluene diisocyanate (TDI) derived viscoelastic foams undergoing compression testing in accordance with D3574, test M, have been observed to recover to its original height at 16 seconds or longer whereas the bulk recovery time is about 3 to 4 seconds. To an end user, this discrepancy is significant and is directly related to feel. The feel experienced by the end user would be that of a viscoelastic foam having a relatively fast recovery time since that is how the bulk of the viscoelastic material behaved, which is markedly different compared to viscoelastic foam having a constant recovery time from the compressed thickness all the way back to the original height, which would have a somewhat softer feel even though total recovery times are substantially identical. Because of this potential for error in total recovery time, the current testing methodology makes it difficult to compare viscoelastic foams with any degree of accuracy. The small impression experienced by some types of viscoelastic foam around the compression plate tricks the ASTM methodology into believing the foam is still recovering.
Moreover, current ASTM testing methodology is generally limited by the time it takes for the compression plate to retract. Useful information below 3 seconds using current ASTM methodology is difficult to obtain. The ASTM retraction rate is such that it takes the compression plate after compression to the compressed height about 3 seconds or more to retract. This prevents accurate measurements for viscoelastic foam materials with response times less than 3 seconds.
Finally, current foam compression testing methodology does not measure the response path. Instead, the compression test simply provides a value for total recovery time after compression at 25 percent of original thickness for a defined amount of time before release of the compression plate. Total recovery time is recorded once the foam reaches the original height and contacts the compression plate. There is no indication as to whether the recovery response was linear or non-linear.